1. Field of the Invention
The present invention is generally related to instruments which can measure the surface energy of a material and, more particularly, to a compact surface energy meter which can perform surface energy measurements in an automated fashion.
2. Description of the Prior Art
Knowledge of the surface energy of a material is particularly important in the plastics field. For example, the ability to print or coat a material is highly dependent on the surface energy of the material. In addition, the surface energy of the material will determine its suitability for medical or biotechnology applications. The surface energy of a material is a measure of the thermodynamic energy needed to increase surface area and is measured in dynes/cm. For historical reasons, the same parameter in liquids is called surface tension, rather than surface energy. Surface energy measurements have been routinely made for several years by a variety of different techniques.
One method of making surface energy measurements involves optically determining the contact angle of a drop of pure water on the surface of the material of interest. Since the surface tension of pure water is 72 dynes/cm at 25.degree. C., knowledge of the contact angle can yield the surface energy of the material from the Girifalco-Good-Fowkes-Young equation which is set forth in Equation 1 below: ##EQU1## The Girilalco-Good-Fowkes-Young equation is a thermodynamic expression which relates solid surface energy .gamma..sub.S to liquid tension .gamma..sub.L and contact angle .theta., with a small correction term for vapor pressure .pi..sub.SV, and is discussed in J. Phys. Chem. 61: 904 (1957), which is herein incorporated by reference.
Another method of making surface energy measurements involves the use of a plurality of fluids, each with a known surface tension, where the technician sequentially smears different fluids on the surface of the material of interest. The surface energy of the material is determined when one of the fluids is found to just form a continuous film.
Still another method of making surface energy measurements involves dipping the material into a fluid of known surface tension and then weighing the material. The weight of fluid which adheres to the surface of the material is then used to provide a measure of the surface energy of the material.
A problem with the prior art methods of making surface energy measurements is that they rely heavily on the judgement and ability of skilled technicians. Providing a surface energy meter which requires minimal training for use, but which can provide accurate and reliable data, would be very beneficial in the plastics field.